1. Field of the Invention
The present invention relates to a voltage-nonlinear resistor element and a method for producing the voltage-nonlinear resistor element.
2. Description of the Related Art
Voltage-nonlinear resistor elements (i.e., varistor elements), which are elements having a structure including a pair of electrodes and a voltage-nonlinear resistor interposed therebetween, have been widely used as elements that protect an electronic circuit or the like from an abnormal voltage in various sensors such as humidity sensors and temperature sensors. An example of voltage-nonlinear resistor elements is disclosed in Patent Literature 1, the element including a portion in which a region composed primarily of zinc oxide and a region including a bismuth-alkaline earth-copper composite oxide are joined to each other. The above voltage-nonlinear resistor element is produced by the following production method. A zinc oxide powder is formed into a compact by a common shaping method. The compact is fired at 1250° C. for 2 hours in air. Both surfaces of the resulting sintered compact are polished. Specifically, one of the surfaces of the sintered compact is mirror-polished with an alumina fine powder. After the polished sintered compact has been cleaned with an organic solvent to a sufficient degree, a sputtered film composed of a Bi-alkaline earth-Cu oxide is formed on the mirror-polished surface of the zinc oxide sintered compact with a high-frequency sputtering apparatus. Then, a heat treatment is performed in an oxygen atmosphere at 870° C. for 24 hours.